vhost-scsi: prevent using uninitialized vqs

vhost-scsi: prevent using uninitialized vqs

There are 3 virtqueues (ctrl, event and cmd) for virtio scsi device,
but seabios will only set the physical address for the 3rd one (cmd).
Then in vhost_virtqueue_start(), virtio_queue_get_desc_addr()
will be 0 for ctrl and event vq.

In this case, ctrl and event vq are not initialized.
vhost_verify_ring_mappings may use uninitialized vhost_virtqueue
such that vhost_verify_ring_part_mapping returns ENOMEM.

When encountered this problem, we got the following logs:

    qemu-system-x86_64: Unable to map available ring for ring 0
    qemu-system-x86_64: Verify ring failure on region 0

Signed-off-by: Forrest Liu <forrestl@synology.com>
Signed-off-by: yuchenlin <yuchenlin@synology.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>